Solid state memory devices that use a structural phase-change material as the data storage mechanism (referred to here simply as ‘phase-change memories’) offer significant advantages in both cost and performance over conventional charge storage based memories. The phase-change memory is made of an array of constituent cells where each cell has some structural phase change material to store the cell's data. This material may be, for instance, a chalcogenide alloy that exhibits a reversible structural phase change from amorphous to crystalline. A small volume of the chalcogenide alloy is integrated into a circuit that allows the cell to act as a fast switching programmable resistor. This programmable resistor can exhibit greater than 40 times dynamic range of resistivity between a relatively crystalline phase (low resistivity) and a relatively amorphous phase (high resistivity). The data stored in the cell is read by measuring the cell's resistance. The chalcogenide alloy cell is also non-volatile.
FIGS. 1A and 1B illustrate an example of a phase change memory cell. As shown, the memory cell 10 includes a phase change material 12 disposed between a bottom electrode 14 and a top electrode 16. A bottom electrode contact 18 provides for electrical contact between the bottom electrode 14 and the phase change material 12. A transistor 20 selectively supplies a current to the memory cell 10 to selectively change the state of the phase change material. FIG. 1A illustrates the state of the phase change material 12 when reset (i.e., when in the amorphous state), and FIG. 1B illustrates the state of the phase change material 12 when set (i.e., in the crystalline state). As will be appreciated from FIG. 1A, the phase change material 12 is not completely changed to the amorphous state when reset, and the phase change material 12 may not be complete changed to the crystalline state when set.
A conventional technique for programming a phase-change memory cell is to apply a rectangular pulse of current (having a constant magnitude throughout the pulse) to the cell 10, at a voltage greater than a switching threshold for the phase change material 12, which leaves the cell 10 in the reset state (the phase change material 12 is relatively amorphous and has high resistivity). To change the state to a set state (the phase change material 12 is relatively crystalline and has low resistivity), a rectangular lower current pulse, also at a voltage greater than the switching threshold, is applied to the memory cell 10. The reset pulse has a higher magnitude of current than the set pulse so that the temperature of the phase change material 12 is raised to an amorphizing temperature, before the phase change material 12 is rapidly cooled down or quenched by the very sharp decrease in current at the trailing edge of the reset pulse; thereby leaving the phase change material 12 in the amorphous phase. To change into the crystalline phase, the phase change material 12 can be heated to a temperature, which is lower than the amorphizing temperature, using a rectangular current pulse of smaller magnitude, and then rapidly cooled down again, this time leaving the phase change material 12 in the crystalline (low resistance) phase. Here, the set pulse is considerably longer than the reset pulse.
It is also known to change the phase change material 12 into the crystalline phase by heating the phase change material up to the amorphizing temperature and slowly reducing the applied current to reduce the temperature of the phase change material 12. As the temperature of the phase change material 12 slowly decreases, the phase change material 12 crystallizes. This method of setting the memory cell also requires a significant amount of time.